pH-dependent Raman study of pyrrole and its vibrational analysis using DFT calculations

Raman spectra of pyrrole in aqueous medium at different pH values, 2.5, 5.5, 7.5 and 10.5 were recorded in the two spectral regions, 1,040-1,160 cm(-1) and 3,300-3,360 cm(-1) and pH dependence of the linewidth, peak position and intensity of the Raman bands corresponding to the ring breathing and symmetric nu(N-H) stretching modes were examined. A linear pH dependence of the peak positions for the ring breathing mode and a maximum at nearly neutral pH (7.5) for the symmetric nu(N-H) normal mode is observed, whereas the linewidth (FWHM) shows almost no variation with the change of pH. A slight decrease in the wavenumber position of the nu(N-H) mode at pH value >7.5 indicates that the influence of deprotonation is small, which results from a weak interaction between the reference molecule and the surrounding environment. The density functional theory (DFT) calculations were made primarily to obtain the optimized geometry and vibrational spectra of pyrrole in the ground electronic state using B3LYP functional and the highest level basis set 6-311++G(d,p). The assignments of the normal modes of pyrrole were made on the basis of potential energy distribution (PED). The calculations were also performed on protonated and deprotonated structures of pyrrole.